Both men claim to be the rightful leader and are contesting seats again.

Some 3,500 candidates are contesting just over 100 parliamentary seats.

Australia on Sunday welcomed the start of the elections.

"It's very good news that these elections are happening on time," said Richard Marles, Australia's parliamentary secretary for Pacific Island affairs.

He said the logistical difficulties meant the initial reports from polling stations were mixed.

But he added: "I think we're very pleased to see them going ahead, it's been a difficult period in PNG politics for the last 12 months. Not everything about this election is going to be perfect."

The BBC's Phil Mercer in Sydney says a small team of Commonwealth officials has been deployed to monitor the elections, but its task is enormous in such a vast, rugged country.

He says this has been a tumultuous year in the resource-rich Melanesian nation of 6.3m, with the country having two prime ministers, two police commissioners and two heads of the armed forces serving at the same time.